Experimental and modeling analyses of COD removal from industrial wastewater using the TiO2–chitosan nanocomposites

In the present study, titanium oxide (TiO2) nanoparticles, chitosan, and several nanocomposites containing different mass dosages of TiO2 and chitosan have been applied as the adsorbent for COD removal from the industrial wastewater (Bouali Sina Petrochemical Company, Iran). The FESEM, XRD, and FTIR tests have been employed to characterize TiO2 nanoparticles, chitosan, and fabricated nanocomposites. Then, the effect of adsorption parameters, including TiO2–chitosan mass ratio (1:1, 1:2, and 2:1), adsorbent content (0.25–2.5 g), temperature (20–50 °C), pH (3–11), solution volume (100–500 mL), and contact time (30–180 min) on the COD reduction has also been monitored both experimentally and numerically. The Box–Behnken design of the experiment approves that TiO2–chitosan (1:1), adsorbent content of 2.5 g, temperature = 20 °C, pH 7.4, solution volume of 100 mL, and contact time = 180 min are the condition that maximizes the COD removal (i.e., 94.5%). Moreover, the Redlich–Peterson and Pseudo-second order models are the best isotherm and kinetic scenarios to describe COD removal’s transient and equilibrium behaviors. The maximum monolayer COD adsorption capacity of the TiO2–chitosan nanocomposite is 89.5 mg g−1. The results revealed that the industrial wastewater COD is better to remove using the TiO2–chitosan (1:1) at temperature = 20 °C.

Determination of COD of solutions. The performance of the fabricated TiO 2 -chitosan nanocomposites for reducing the wastewater COD was measured using the standard procedure of HACH. Indeed, the closed reflux method 56 in a HACH COD reactor (DRB200, Hach Co., Loveland) containing K 2 Cr 2 O 7 (potassium dichromate) reagent has been applied to measure the wastewater COD ranging from zero to 1500 mg L −1 . Then, 2 mL aliquots were added to the COD vials at 150 °C for 2 h. The COD vials were cooled to room temperature and titrated with ferrous ammonium sulfate (molarity = 0.05). The solution pH adjusts using H 2 SO 4 (molarity = 0.1) or NaOH Scientific Reports | (2022) 12:11088 | https://doi.org/10.1038/s41598-022-15387-0 www.nature.com/scientificreports/ (molarity = 0.1). After conducting the adsorption tests, the adsorbent separates from the extract through 10 min of centrifugation at 4000 rpm (Denley BS400 machine, UK). The TDS (total dissolved solids), initial pH, and COD of Bouali Sina Petrochemical Company wastewater are 574 mg L −1 , 7.3, and 0.97 g L −1 , respectively. Equation (1) expresses the mathematical formulation of the COD removal 57 .
where C i and C o stand for the initial and final COD concentrations, respectively.
where A 0 , A k , A kk , A kz are the model's coefficients. X k , X k 2 , and X k X z are the three combinations of the independent variables (linear, quadratic, and interactive). Table 1 summarizes the output of applying the design of the experiment to the adsorption parameters. This table also reports the experimentally-measured COD removal values (see "Effect of operating conditions on the COD removal" section), and their counterpart predicted values by a polynomial model (see "Statistical analyses of the adsorption experiments" section).
Kinetic and isotherm models. In the optimum condition of the adsorption process, the temperature effect (20-50 °C) on the COD reduction of the considered industrial wastewater was also investigated. The performance of TiO 2 , chitosan, and synthesized nanocomposites with different mass ratios of 1:1, 1:2, and 2:1 TiO 2 and chitosan on the COD reduction was examined. Two famous kinetic models [i.e., pseudo-1st-order (Eq. 3) 59 A kz X k X z where q e and q t present the COD removal capacity of an adsorbent at the equilibrium state and time t, respectively. Freundlich, Redlich-Peterson, and Langmuir isotherms have also been checked to model the equilibrium measurements of the COD removal.

Results and discussion
Adsorbent characterization tests. FESEM. The FESEM images of the TiO 2 nanoparticles, chitosan, and TiO 2 -chitosan nanocomposite are presented in Fig. 1a-c, respectively.
These characterization tests show that the TiO 2 , chitosan, and TiO 2 -chitosan nanocomposite are homogenous and have an average particle size of 30, 35, and 40 nm. It can also be seen that the chitosan and TiO 2 were appropriately dispersed in the structure of the TiO 2 -chitosan nanocomposite. The morphology of the TiO 2 -chitosan has a particle size ranging from 15 to 60 nm. In the TiO 2 nanoparticles' FTIR spectrum, the absorption spectra at 3737, 3231, 2359, and 1642 cm −1 are associated with the hydroxyl groups. The observed band at 650 cm −1 revealed the existence of the TiO 2 compound.
Moreover, the characteristic bands of the chitosan and TiO 2 can be easily detected in the TiO 2 -chitosan nanocomposite's FTIR spectrum. No significant differences are observable in the FTIR spectra of the chitosan, TiO 2 , and the synthesized TiO 2 -chitosan nanocomposite. It implies that the TiO 2 addition into the chitosan structure produces no changes in the chitosan chemical structure. These observations approved that the TiO 2 was physically loaded in the chitosan structure.
Effect of operating conditions on the COD removal. The effects of four influential factors (i.e., adsorbent content, contact time, pH, and solution volume) on the industrial wastewater COD removal have been measured at three working levels. Figure 4a shows the pH impact on the COD removal efficiency of the TiO 2 - www.nature.com/scientificreports/ chitosan nanocomposite. This figure states that increasing the solution pH up to 7 increases the COD removal, and after that, the COD removal efficiency of TiO 2 -chitosan nanocomposite decreases. Higher H + ion concentration in the acidic solution (pH of lower than 7) neutralizes the negative charge of the TiO 2 -chitosan surface and reduces the COD removal efficiency by the ion exchange. On the other hand, high OHion concentration in the alkaline/basic solution (pH of higher than 7) prevents the diffusion of organic materials into the TiO 2chitosan pores and decreases the COD removal 62 . Furthermore, the surface charge of the adsorbent depends on the solution pH. The zero charge point of a TiO 2 in water is at pH ~ 6. At the alkaline range of pH, the positive surface charge of the adsorbent may be responsible for decreasing the COD removal efficiency of the TiO 2 -chitosan nanocomposite 40 . Similar results were also reported by other researchers 63,64 . The optimum pH value of 7 has been reported for maximizing the COD removal efficiency of some adsorbents for wastewater treatment of the coffee 63 and sugar 64 processing companies. The influence of wastewater-nanocomposite contact time at three levels on the COD removal performance of the TiO 2 -chitosan adsorbent is depicted in Fig. 4b. It can be concluded that the adsorption capacity of the TiO 2 -chitosan nanocomposite increases by increasing the contact time. COD adsorption using the TiO 2 -chitosan adsorbent experiences the equilibrium state at contact time = 180 min. More than 90% of the total COD has been adsorbed at the first 105 min of the contact time. The sharp variation of the COD removal during the first 105 min of contact time is associated with the high number of active sites available at the TiO 2 -chitosan surface. After saturating surface-active sites, the organic matters require more time to diffuse through the TiO 2 -chitosan pores and adsorb on the pore walls of the nanocomposite. After 180 min of contact time, all the internal/external active sites of the TiO 2 -chitosan nanocomposite have been occupied, and the equilibrium state is reached. A similar trend has been reported for water removal from 2-dimethylaminoethylazide using calcium chloride and NaA zeolite 65 . Figure 4c shows the influence of adsorbent content on the COD removal from industrial wastewater. This figure explains that increasing the adsorbent content increases the available active sites for pollutant adsorption and enhances the COD removal efficiency of the utilized nanocomposite. This figure also shows that the COD removal rate decreases by increasing the adsorbent content (> 1.375 g). Indeed, decreasing the available organic matter to adsorb on the active nanocomposite sites reduces the COD removal rate of a high nanocomposite dosage.
The impact of the effluent volume of wastewater/solution on the COD removal efficiency of the fabricated nanocomposite is illustrated in Fig. 4d. This figure indicates that increasing the solution volume increases the number of organic matters, rapidly saturates the available active sites of the nanocomposite, and decreases the COD removal. The low performance of the TiO 2 -chitosan nanocomposite for efficiently removing the COD of 500 mL of effluent volume is related to the rapid saturation of adsorbent sites. Indeed, the lower COD removal efficiency achieved for the high than the low wastewater effluent volume is connected to the higher COD needed to be adsorbed/removed by the same number of active sites. www.nature.com/scientificreports/ The COD removal ability of TiO 2 nanoparticles, chitosan, and synthesized nanocomposites with mass ratios of 1:1, 1:2, and 2:1 of TiO 2 and chitosan have been compared in Fig. 5. This graph shows that the maximum COD removal of 80% can be achieved by helping the TiO 2 -chitosan (1:1) adsorbent at pH 7, contact time 180 min, adsorbent content 2.5 g, and 300 mL solution volume. The COD removal ability of the utilized adsorbents has an order of TiO 2 -chitosan 1:1 (80%) > TiO 2 -chitosan 1:2 (76%) > TiO 2 -chitosan 2:1 (73%) > TiO 2 (69%) > chitosan (65%). Therefore, the TiO 2 -chitosan with an equal mass ratio is the best adsorbent for COD removal from industrial wastewater.
Dependency of the COD removal efficiency of the TiO 2 -chitosan (1:1) nanocomposite 100 mL of the effluent solution (pH 7.4, adsorbent content = 1.375 g, contact time = 105 min) has been illustrated in Fig. 6. This figure approves the negative effect of temperature on the COD removal efficiency of the TiO 2 -chitosan nanocomposite. It means that the TiO 2 -chitosan adsorbent has the highest tendency to remove the wastewater COD at low temperatures. This behavior may be associated with increasing the internal energy of pollutants that helps them to detach from the adsorbent surface and escape into the solution bulk. Exothermic adsorption may be considered the next responsible for this observation 66 . Thus, both physical and ion exchange are possible to involve in the COD sorption process using the TiO 2 -chitosan nanocomposite. This observation has also been reported by other scientists 67,68 . Table 2 summarizes the results of ANOVA (analysis of variance) performed to inspect the significance probability (p-value) of influential variables on the COD removal efficiency of the nanocomposite. Those independent variables with p < 0.05 at the 95% confidence interval significantly impact the COD removal 69 . The significant variables are necessary to include in the full   71 . Table 3 reports the ANOVA results for only the significant variables (p < 0.05). Equation (5) presents the polynomial model developed to predict COD removal from the significant variables.

Statistical analyses of the adsorption experiments.
where X 1 , X 2 , X 3 , and X 4 stand for the solution pH, contact time (min), adsorbent content (g), and effluent solution volume (mL), respectively. Comparing the lack of fit before (0.252) and after (0.224) elimination of the insignificant parameters reveals considerable improvement in the model prediction accuracy. A relatively high achieved correlation coefficient (R 2 > 0.99) implies an excellent compatability between the experimental COD removal values and their counterpart predictions by the developed model. Equation (6) 73 . The crossplot of the predicted COD removal (FITS1) versus their associated experimental measurements has been exhib-  www.nature.com/scientificreports/ ited in Fig. 7b. It can be easily concluded that slight deviations exist between the experimental COD removal values and the model predictions. The high correlation coefficient value (R 2 = 0.999) approves that the constructed model accurately approximates the experimentally-measured COD data.
Optimizing the adsorption parameters. It is possible to locate the optimized values of the involved independent variables by solving Eq. (5). The optimum values of the solution pH, contact time, adsorbent content, and effluent solution volume are 7.4, 180 min, 2.5 g, and 100 mL, respectively. In this optimum condition, the maximum COD removal efficiency of the TiO 2 -chitosan nanocomposite is 93.67%. The experimental value of the COD removal under the optimum condition (i.e., 94.5%) is also in excellent agreement with the predicted optimized value.
Monitoring the combined effect of independent variables. Figure   www.nature.com/scientificreports/ greater period available for organic matter to absorb on the nanocomposite surface and diffuse in its pores. The simultaneous effects of pH and adsorbent content (Fig. 8b) and pH and effluent solution volume (Fig. 8c) on the wastewater COD removal reveal that the optimum pH value is about 7. The couple effects of pH and contact time (Fig. 8a), adsorbent dosage and contact time (Fig. 8d), and contact time and volume solution (Fig. 8e) indicate that two different mechanisms govern the COD adsorption efficiency of the utilized nanocomposite over the time. In the first stage (up to 105 min), the fast COD adsorption may be related to the adsorption of organic matters on the external surface of the TiO 2 -chitosan nanocomposite. In the second stage, the organic matter diffuses through the TiO 2 -chitosan composite pores and throats and absorbs on the internal active sites. Furthermore, enhancing the COD removal by the TiO 2 -chitosan nanocomposite dosage can be related to increasing the available surface area and active sites for adsorbing pollutants (Fig. 8b,d,f). The simultaneous effect of volume and pH, adsorbent dosage, and contact time have been presented in Fig. 8c,e,f. These graphs state that increasing the effluent wastewater volume negatively affects the COD removal efficiency of the nanocomposite. An increase in the effluent wastewater volume increases the pollutant concentration, rapidly saturates the active sites, and reduces the COD removal ability of the nanocomposite.
Kinetic studies. Referring to Eqs. (3) and (4), the adjustable constants of the pseudo-1st-order and pseudo-2nd-order kinetic equations are shown by k 1 and k 2 , respectively. Table 4 introduces the adjusted constants of the considered kinetic models, experimental and calculated values of the COD adsorption capacity at the equilibrium state, and the observed correlation coefficients. Since the pseudo-2nd-order has a higher correlation coefficient than that of the pseudo-1st-order kinetic approach, the earlier better describes the transient behavior of the COD removal by the TiO 2 -chitosan nanocomposite. Furthermore, the COD adsorption capacity obtained by the pseudo-2nd-order has a higher compatibility with the experimental measurements (R 2 = 0.993) than those provided by the pseudo-1st-order kinetic model (R 2 = 0.970). Thus, the pseudo-2nd-order kinetic model is chosen for modeling the transient behavior of the wastewater COD removal using the TiO 2 -chitosan adsorbent. This is close to the results discussed by Prakash et al. 35 .  where k F and n are Freundlich's model constants. q m and b show the coefficients of the Langmuir model. P, α, and β stand for the Redlich-Peterson model parameters. Adjusted parameters of the selected isotherms for describing the equilibrium COD removal using the TiO 2 -chitosan composite have been reported in Table 5. This table also introduces the numerical values of the observed correlation coefficients. It can be seen that the Redlich-Peterson isotherm model has the highest R 2 value (i.e., 0.991), and the Freundlich isotherm possesses the smallest R 2 (i.e., 0.970). Since the adjusted value of the β (for the Redlich-Peterson isotherm) is close to 1, it can be concluded that the monolayer COD adsorption by the TiO 2 -chitosan nanocomposite is the predominant scenario.
Cost analysis. The cost of chitosan and TiO 2 nanopowder was obtained as presented in Table 6. According to

Conclusion
This research studied industrial wastewater treatment using TiO 2 nanoparticles, chitosan, and TiO 2 -chitosan nanocomposite from experimental and numerical points of view. The considered adsorbents have been characterized by the XRD, FTIR, and FESEM tests. The XRD pattern proved that the synthesized TiO 2 -chitosan nanocomposite preserves the characteristic structure of TiO 2 nanoparticles. Analyzing the FTIR spectra approved that the TiO 2 nanoparticles have been physically loaded in the chitosan structure. The FESEM tests confirmed that the TiO 2 -chitosan nanocomposite has a particle size ranging from 15 to 60 nm. The impact of solution pH, temperature, adsorbent mass and composition, contact time, and effluent solution volume on the COD removal has been monitored using the experimental and modeling analyses. The optimum condition for the considered process (pH 7.4, contact time = 180 min, nanocomposite mass = 2.5 g, and wastewater effluent volume = 100 mL) has been determined using the Box-Behnken design of the experiment. Furthermore, results approved that the TiO 2 -chitosan (1:1) at the lowest allowable temperature is better to employ for industrial wastewater treatment. The maximum experimental and calculated COD removal efficiency of the TiO 2 -chitosan nanocomposite is 93.67% and 94.5%, respectively. The Redlich-Peterson isotherm and Pseudo-2nd-order kinetic models showed (7) q e = k F C 1/n e (8) q e = PC e / 1 + αC β e (9) q e = q m bC e /(1 + bC e )